The present invention relates generally to the field of lenticular imaging, and in particular, to a method of forming a lenticular sheet for use in forming a substantially ghost-free lenticular image.
Lenticular images, which are well known, comprise a plurality of interlaced scenes that are printed directly on or affixed adjacent to a generally planar bottom surface of a lenticular sheet. The opposing top surface of the lenticular sheet typically comprises a plurality of convex lenticles that are parallel to each other and the interlaced scenes printed on the bottom surface of the lenticular sheet. When viewed in cross-section, the top surface of each lenticle in the lenticular sheet typically has either a circular or an elliptical profile.
The individual lenticles act as lenses that are intended to allow an observer to view only a single interlaced scene at a particular viewing angle. As the viewing angle changes, the viewer should view a different interlaced scene. Because the scene observed changes as a function of viewing angle, it is possible to dissect and sequentially print interlaced scenes to form lenticular images that appear to an observer as a motion picture or video clip.
A number of means are known for forming lenticular sheets. These means include machining, platen pressing, injection or compression molding, extrusion embossment, and casting. For reasons of cost and manufacturing ease, lenticular sheets are typically formed from polymeric materials. But other materials such as glass can be used.
Machining is used primarily to manufacture large, one-of-a-kind, lenticular sheets from thick plastic sheets or plates. Milling machines or lathes equipped with diamond tipped cutting tools can be used to fabricate lenticular sheets of this type. It will be appreciated that machining is a slow and costly process, and thus it is not particularly suitable for high volume production of lenticular sheets.
A platen press can be used to stamp or emboss a lenticular pattern into a polymeric material. In such applications, the polymeric material typically comprises a thermosetting material. The temperature of the thermosetting polymeric material is raised to soften the material so that it conforms to the embossed surface. The temperature is then reduced to harden the polymeric material so that it retains the lenticular pattern when removed from the platen press. Like machining, this method is relatively slow and expensive. Furthermore, the size of the lenticular sheets that can be produced by this process is limited by the size of the press. Accordingly, this method is not particularly suitable for high volume production of lenticular sheets. The same disadvantages are present with injection or compression molding techniques, wherein the lenticular sheet is formed by injecting or compressing a molten polymer into a mold that includes a lenticular pattern on one side.
By far, the most common method of fabricating lenticular sheets is by extrusion embossment. Typically, extrusion embossment involves the use of an embossed roll, that may or may not be chilled. A molten polymeric material, which is typically thermoplastic rather than thermosetting, is extruded onto the embossed roll where it conforms to and thus is imprinted with the lenticular relief pattern. As the polymeric material cools, it retains the embossed lenticular pattern and is pulled from the embossed roll.
Although lenticular images have been produced since at least as early as the 1950""s, the quality of most lenticular images has not been particularly good. Most known lenticular images suffer from a phenomenon known within the art as xe2x80x9cghosting,xe2x80x9d which describes an observer""s ability to see more than one interlaced scene at a particular viewing angle. A viewer of a lenticular image that contains xe2x80x9cghostsxe2x80x9d will see phantom scenes within the image. For example, if the lenticular image comprises a series of interlaced scenes whereby the letter xe2x80x9cCxe2x80x9d changes or xe2x80x9cmorphsxe2x80x9d into an image of a cat, a ghost in the form of all or some of the xe2x80x9cCxe2x80x9d image may be observed in the final cat image.
The current practice in the field of lenticular imaging is to try to eliminate the appearance of ghosting and other undesirable artifacts in lenticular images through trial and error. Lenticular film fabricators usually attempt to address ghosting by randomly varying the lenticular sheet by changing the lens frequency or the thickness of the lenticular sheet. Some printers of lenticular images address ghosting by inserting colored strips between interlaced scenes in an attempt to further separate and differentiate them. These trial and error techniques rarely yield substantially ghost-free images and are implemented at high cost. For example, a lenticular sheet fabricator who decides to vary the lens frequency does so by having a new embossing roll manufactured. Embossing rolls are relatively expensive, and without proper guidance, a new embossing roll may not solve the ghosting problem.
A method is needed whereby fabricators of lenticular sheets and printers of lenticular images can determine the proper geometry and/or thickness of a lenticular sheet suitable for use in forming a substantially ghost-free lenticular image for use in a particular lenticular image forming application. Preferably, the method would permit lenticular sheet fabricators to use existing inventories of embossing rolls and other means for forming lenticular sheets, and thus avoid the expense and time needed to obtain other suitable means.
The present invention provides a method of forming a lenticular sheet for use in forming a substantially ghost-free lenticular image. The method according to the invention comprises providing a means for forming a lenticular sheet and forming the lenticular sheet out of a polymeric material using the means such that the lenticular sheet has an actual thickness (da) that is within about xc2x115% of an optimal thickness (do) as determined according to the formula: do=(nsxe2x88x921)r/s(nxe2x88x921), wherein n is the refractive index of the polymeric material, s is the number of interlaced scenes in the lenticular image, and r is the radius of the lenticles formed using the means. Suitable means for forming a lenticular sheet according to the method of the invention include machining, platen pressing, injection molding, compression molding, extrusion embossment, and casting, with extrusion embossment being presently most preferred.
For some lenticular imaging applications, it is desirable to form a lenticular sheet having a predetermined thickness da. In such instances, the method of the present invention comprises providing a means for forming a lenticular sheet and forming the lenticular sheet out of a polymeric material using the means such that the lenticles have an actual radius (ra) that is within about xc2x115% of the optimal radius (ro) determined according to the formula: ro=(nxe2x88x921)sda/(nsxe2x88x921), wherein n is the refractive index of a polymeric material from which the lenticular sheet is to be formed, s is the number of interlaced scenes in the lenticular image, and da is the predetermined actual thickness of the lenticular sheet.
In some lenticular imaging applications it is possible to obtain substantially ghost-free lenticular images by selecting a polymeric material having a proper refractive index. Accordingly, in another embodiment of the method of the present invention, the method comprises providing a means for forming a lenticular sheet and forming the lenticular sheet from a polymeric material using the means, wherein the polymeric material has an actual refractive index (na) that is within xc2x13% of an optimal refractive index (no) as determined according to the formula: no=(raxe2x88x92sda)/s(raxe2x88x92da), wherein ra is the actual radius of each lenticle in the lenticular sheet, s is the number of interlaced scenes in the lenticular image, and da is the predetermined actual thickness of the lenticular sheet.
In all cases, it is preferable for the lenticular sheet to have an actual frequency (LPIa) in lenticles per inch that is within about xc2x120% of an optimal frequency (LPIo) in lenticles per inch as determined according to the formula: LPIo=Lxcex8min, where L is the predetermined optimal viewing distance of the lenticular image in inches and xcex8min is a predetermined minimum visual acuity value for an observer selected within the range of from about 0.0002 to about 0.0005.
By employing the method of the present invention, lenticular sheet fabricators can readily determine the proper geometry, thickness, and/or refractive index of the polymeric material being used in order to form a substantially ghost-free lenticular image for a particular lenticular image forming application. The method of the present invention eliminates the guesswork and waste incurred in conventional lenticular sheet production and allows for the production of high quality, substantially ghost-free lenticular images.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
The present invention provides a method of forming a lenticular sheet for use in forming a substantially ghost-free lenticular image. Throughout the instant specification and in the appended claims, the term xe2x80x9csubstantially ghost-freexe2x80x9d means that a viewer cannot observe more than one interlaced scene at a particular viewing angle when the lenticular image is viewed at the optimal viewing distance. Notwithstanding the foregoing, it will be appreciated that because an observer""s eyes are spaced apart a distance, and thus are at slightly different viewing angles relative to the lenticular sheet, some very minor artifacts such as faint ghosts may actually be observed by some viewers in a substantially ghost-free lenticular image. This is particularly true when the viewing distance from the observer""s eyes and the lenticular image is relatively small. Any ghosts that can be observed in a substantially ghost-free lenticular image will, however, generally be limited to the adjacent interlaced scene only. Additionally, variations between particular observers"" eyesight can result in one observer being able to detect a small degree of ghosting in a lenticular image that another observer cannot otherwise detect.
Applicants have discovered that in order to obtain a substantially ghost-free lenticular image, it is critical that the magnification of the lenticles of the lenticular sheet be matched, within a reasonable margin of error, to the number of interlaced scenes in the lenticular image. Each lenticle in a lenticular sheet magnifies the image of the material situated directly beneath it. As the viewing angle changes, a different scene is brought into view because the lenticle focuses on a different location. If more than one interlaced scene can be viewed at a particular viewing angle, undesired artifacts such as ghosting are observed. By matching the magnification of the lenticles in the lenticular sheet to the number of scenes present in the image, the lenticular sheet performs its task in an optimum manner. The lenticles of the lenticular sheet magnify the printed image in such a manner so as to create a virtual image that appears to be located behind the lenticular sheet.
One way to control the magnification of the lenticular sheet is to adjust the thickness of the lenticular sheet. The thickness of a lenticular sheet is typically measured using calipers, with the term xe2x80x9cthicknessxe2x80x9d describing the distance from the flat bottom surface of the lenticular sheet to the highest point on the convex top surface of a lenticle. The optimal thickness (do) of a lenticular sheet can be determined according to the following formula:       d    o    =                    (                              n            ⁢                          xe2x80x83                        ⁢            s                    -          1                )            ⁢      r              s      ⁡              (                  n          -          1                )            
wherein n is the refractive index of the polymeric material from which the lenticular sheet is being formed, s is the number of interlaced scenes in the lenticular image, and r is the radius of the lenticles on the lenticular sheet. The radius of a lenticle is defined as being the radius of a circle that would overlap the profile of the lenticle when viewed in cross-section. For lenticles having an elliptical cross-section, the radius is defined as being the radius of a circle that most completely overlaps the profile of the lenticle when viewed in cross-section.
In the best case, it would be desirable to have the actual thickness (da) of the lenticular sheet match the optimal thickness (do) for the particular application. However, it is generally acceptable for the actual thickness (da) to be within xc2x115% of the optimal thickness (do) for the particular application. More preferably, the actual thickness (da) is within xc2x110% of the optimal thickness (do). In the most preferred embodiment of the invention, the actual thickness (da) is within xc2x15% of the optimal thickness (do).
As noted above, it is critical that the magnification of the lenticles of the lenticular sheet be matched, within a reasonable margin of error, to the number of interlaced scenes in the lenticular image. However, this does not necessarily mean that each of the interlaced scenes must be different. It only means that the magnification of the lenticles of the lenticular sheet be matched to the total number of interlaced scenes comprising the lenticular image. To illustrate the foregoing point, suppose a printer desires to print a lenticular image that includes three different scenes or pictures that change as a function of the observer""s viewing angle relative to the lenticular image. For purposes of this illustration, the three different scenes or pictures are represented by the characters A, B and C. There are several options available to the printer. The printer could use a lenticular sheet having lenticles with a magnification suitable for use with three interlaced scenes. In such a circumstance, the area under each lenticle would be printed with the interlaced scenes: ABC. Alternatively, the printer could use a lenticular sheet having lenticles with a magnification suitable for use with a multiple of three interlaced scenes, such as a lenticular sheet having lenticles with a magnification suitable for use with twelve interlaced scenes. In such a circumstance, the area under each lenticle would be printed with the interlaced scenes: AAAABBBBCCCC. In either case, an observer would see a substantially ghost-free lenticular image containing three scenes that smoothly change as a function of the observer""s viewing angle relative to the lenticular sheet.
For some applications, it is sometimes necessary to fabricate the lenticular sheet such that it has a predetermined thickness. In such cases, the magnification of the lenticular sheet can be controlled by adjusting the radius of the lenticles. The optimal radius (ro) of a lenticular sheet can be determined according to the following formula:       r    o    =                    (                  n          -          1                )            ⁢              sd        a                    (                        n          ⁢                      xe2x80x83                    ⁢          s                -        1            )      
wherein n is the refractive index of a polymeric material from which the lenticular sheet is to be formed, s is the number of interlaced scenes in the lenticular image, and da is the predetermined actual thickness of the lenticular sheet.
In the ideal case, it would be desirable to have the actual radius (ra) of the lenticular sheet match the optimal radius (ro) for the particular application. However, it is generally acceptable for the actual radius (ra) to be within xc2x115% of the optimal radius (ro) for the particular application. More preferably, the actual radius (ra) is within xc2x110% of the optimal radius (ro). In the most preferred embodiment of the invention, the actual radius (ra) is within xc2x15% of the optimal radius (ro).
Sometimes, it is also possible to adjust the magnification of the lenticular sheet by selecting a polymeric material having an appropriate refractive index for the particular application or by adjusting the refractive index of the polymeric material using certain additives. The optimal refractive index (no) of a lenticular sheet can be determined according to the following formula:       n    o    =            (                        r          a                -                  sd          a                    )              s      ⁡              (                              r            a                    -                      d            a                          )            
wherein ra is the actual radius of each lenticle in the sheet, s is the number of interlaced scenes in the lenticular image, and da is the predetermined actual thickness of the lenticular sheet.
Clearly, it would be desirable to have the actual refractive index (na) of the polymeric material used to fabricate the lenticular sheet match the optimal refractive index (no) for the particular application. However, it is generally acceptable for the actual refractive index (na) to be within xc2x13% of the optimal refractive index (no) for the particular application. More preferably, the actual refractive index (na) is within xc2x12% of the optimal refractive index (no). In the most preferred embodiment of the invention, the actual refractive index (na) is within xc2x11% of the optimal refractive index (no).
While certainly an option, applicants believe that changing the refractive index of the polymeric material or selecting a polymeric material with a specified refractive index will seldom be the best option for a given lenticular imaging application. Process limitations and other factors such as the need to use specified polymers (which precludes the selection of a different polymer having different refractive index) and/or the need to maintain optical clarity (presently, there are relatively few refractive index altering additives that do not also adversely affect the optical clarity of the polymeric material) can make controlling the magnification of the lenticular sheet via adjustment/selection of the refractive index of the polymer very difficult to accomplish. Furthermore, many of the polymers used in the production of lenticular sheets have refractive indices that are relatively close.
In every case, it is preferable for the frequency of the lenticular sheet (i.e., the number of lenticles per unit of distance, typically inches) to be appropriate for the particular application. Frequency is limited by the minimum visual acuity of the observer and the ability of printers to interlace images. If the frequency is too small, the image may appear to the observer as a series of stripes rather than as the desired image. If the frequency of the lenticular sheet is too large, it will be difficult for the printer to accurately print the interlaced scenes in the lenticular image.
The optimal frequency (LPIo) in lenticles per inch for a lenticular sheet can be determined according to the following formula:       LPI    o    =      1          L      ⁢              xe2x80x83            ⁢              θ        min            
wherein L is the predetermined optimal viewing distance of the lenticular image in inches and xcex8min is a predetermined minimum visual acuity value for an observer. The minimum visual acuity value for humans typically ranges from about 0.0002 to about 0.0005. For ease in making calculations, a minimum visual acuity value of 0.00035 is typically used.
Obviously, it would be desirable to have the actual frequency (LPIa) of the lenticular sheet match the optimal frequency (LPIo) for the particular application. However, it is generally acceptable for the actual frequency (LPIa) to be within xc2x140% of the optimal frequency (LPIo) for the particular application. More preferably, the actual frequency (LPIa) is within xc2x120% of the optimal frequency (LPIo). In the most preferred embodiment of the invention, the actual frequency (LPIa) is within xc2x110% of the optimal frequency (LPIo).
LPIa is permitted to deviate from LPIo over a relatively broad range to account for the tendency of human observers to move closer to or farther away from lenticular images than the distance at which they were intended to be viewed by the designer. For example, a lenticular image that is placed in the aisle of a grocery store might have a predetermined optimal viewing distance of three feet, being roughly half the width of an aisle in a typical grocery store. In practice, however, observers viewing the lenticular image might actually view the image from a distance that is xc2x11 foot from the optimal location due to the curiosities of the observers or variances in the width of aisles in grocery stores. This means that the uncertainty in viewing distance can be as much as 33% or more, which is why a relatively wider range for LPIa is acceptable.
The method according to the present invention can be used with any means for forming a lenticular sheet. Suitable means include, for example, machining, platen pressing, injection molding, compression molding, extrusion embossment, and casting. Extrusion embossment is the presently most preferred means for use in practicing the method of the invention due to considerations such as cost and ease of manufacturing.
One of the advantages of the method according to the present invention is that it allows lenticular sheet fabricators to create an inventory of embossment rolls or other lenticular sheet forming means that can be used to produce lenticular sheets suitable for use in forming substantially ghost-free lenticular images for specified applications. The method avoids the costs, time, and waste of prior art trial and error methods of addressing ghosting.
The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims: